Scout invitations were stored in guild_applications with type 'invited',
forcing the scout list response to use charID as the invitation ID — a
known hack that made CancelGuildScout semantically incorrect.
Introduce a dedicated guild_invites table (migration 0012) with a serial
PK. The scout list now returns real invite IDs and actual InvitedAt
timestamps. CancelGuildScout cancels by PK. AcceptInvite and DeclineInvite
operate on guild_invites while player-applied applications remain in
guild_applications unchanged.
